1. Field of the Invention
The present invention relates to a small antenna device having a patch antenna structure, and more particularly, to an antenna apparatus that is also referred to as a metal plate patch antenna where a radiating conductor plate is composed of a metal plate.
2. Description of the Related Art
Generally, a metal plate patch antenna where a radiating conductor plate is composed of a metal plate has an advantage in that it can be manufactured at a low cost as compared to a patch antenna in which a radiating conductor layer is patterned on one surface of a dielectric substrate. In such a metal plate patch antenna, since the radiating conductor plate is arranged above a ground conductor with an air layer interposed therebetween, the radiating conductor plate is generally supported by a supporting member made of a dielectric material (for example, see Japanese Unexamined Patent Application Publication No. 2002-237714 (page 2, FIG. 6)).
FIG. 6 is a sectional view illustrating an example of a conventional metal plate patch antenna. As shown in FIG. 6, a metal plate patch antenna 1 is made up of a ground conductor 3 patterned on an insulating substrate 2, a radiating conductor plate 4 composed of a metal plate arranged above the ground conductor 3 with a predetermined gap therefrom, and four supporting members 5 made of a dielectric material standing on the ground conductor 3. Four corners of the radiating conductor plate 4 having a substantially square shape are supported by four pillar-shaped supporting members 5. Further, a conductive line 6 is connected to a feeding point of the radiating conductor plate 4. The conductive line 6 is inserted through a through-hole 7 passing through the ground conductor 3 and insulating substrate 2 to connect to an antenna circuit (not shown). In the metal plate patch antenna 1 having the above-mentioned structure, since the supporting members 5 made of a dielectric material are interposed between the ground conductor 3 and an outer circumferential portion of the radiating conductor plate 4 which has an intensive electric field, the size of the radiating conductor plate 4 can be decreased by using a wavelength shortening effect by a dielectric material.
The above-mentioned conventional metal plate patch antenna 1 has an advantage in that the size of the radiating conductor plate 4 can be deceased. However, there is a problem because the antenna efficiency deteriorates from the dielectric loss caused by the supporting member 5. Further, in the conventional metal plate patch antenna 1, since four supporting members 5 made of a dielectric material are interposed between the ground conductor 3 and the radiating conductor plate 4, the material and assembling cost are increased, so that the antenna cannot be manufactured at a low cost.